Sufferers of hearing loss are, in many cases, able to ameliorate the effects of their hearing loss by the use of hearing devices that amplify surrounding sound signals, couple sound to the inner ear in non-acoustic ways, and/or directly stimulate auditory nerves in the recipient's cochlea, for example, using electrical stimulation.
Examples of such devices include hearing aids, which amplify surrounding signals and provide this to the recipient in the form of amplified acoustic signals (hearing aids may also be implantable), Direct Acoustic Cochlear Stimulation (DACS) systems which convert surrounding sound signals into mechanical actuation signals which stimulate the recipient's cochlea directly, Bone Anchored Hearing Aids (BAHA) which transmit sound from the skull to the cochlea via bone conduction, and cochlear implant systems which convert surrounding sound signals into electrical stimulation signals which are applied directly to auditory nerves inside the recipient's cochlea or inner ear. There are also hybrid systems which provide both mechanical (or acoustic) and electrical stimulation.
In some cases, a recipient will have a hearing device in each ear. This is known as a bilateral system. A bilateral system may consist of a cochlear implant in one ear and a conventional hearing aid in the other ear, a cochlear implant in both ears, a hearing aid in both ears, a hybrid system in both ears, a hybrid system in one ear and a cochlear implant in the other ear, or a hybrid system in one ear and a hearing aid in the other ear. Combinations in which a different system is used in the two ears is known as a bimodal bilateral system.
Even though bilateral hearing device users or recipients will have more chance of detecting the direction of sound than a unilateral user or recipient, bilateral users still suffer from very poor localisation skills compared to normal hearing people. In normal hearing people, location of a sound source is accomplished using a number of different strategies, including using cues such as interaural level difference (ILD), in which the level or magnitude of sound is different in each ear depending upon the location of the source of the sound; and interaural time difference (ITD), in which the sound arrives at each ear at different times, depending upon the location of the source of the sound. Because of the way hearing devices process sound artificially, the usefulness of these cues may be lost or diminished in device users or recipients, making it difficult for the recipient to identify or locate the source of a sound. Furthermore, knowledge of the location of the source of a sound can often assist the recipient in understanding speech in noise.